Are you sure that is the case? I have seen people posting in do follow blogs again and again and isn't it mainly because they get more backlinks? I think that is the case. Anyhow, if you get too many backlinks from a single website, then, I suppose, it doesn't get any value.
It won't hurt, but it won't help you either. Chances are those links will be low-quality directory submissions, blog comments and forum signatures anyway. Other than being a spammy site, the only thing I can think of that would bend your site over the proverbial table is if the site you were getting your links from was to suddenly go off-line for an extended period of time.
In my opinion, I feel that it will help, example a blog, monthly also mention on your website and link back to you, each topic are different content, but the content related to your website. Futhermore, during google crawl those blog, the spider will easily come your website with the back link provide from there. my opinion might be wrong. correct me if you have different view
Those blog comments won't help one iota if the link to your Web site has had the rel="nofollow" attribute applied to it. And even if it didn't, chances are that blog entry would have such a low toolbar PageRank (0-2) or so many links leading away from it that whatever benefit you would have would be negligable at best, non-existant at worst. You want to get the most from another blog? Get mentioned in the blog post and make sure that link DOES allow the search engines to follow it.